gmp: split gmpxx devel stuff into its own subpkg.

This fixes "./xbps-src bootstrap" in void.

srcpkgs/gmp/template

@@ -1,7 +1,7 @@
 # Template build file for 'gmp'
 pkgname=gmp
 version=6.0.0
-revision=2
+revision=3
 bootstrap=yes
 build_style=gnu-configure
 configure_args="--enable-cxx"
@@ -14,6 +14,8 @@ homepage="http://gmplib.org/"
 distfiles="https://gmplib.org/download/gmp/gmp-${version}a.tar.xz"
 checksum=9156d32edac6955bc53b0218f5f3763facb890b73a835d5e1b901dcf8eb8b764
 
+subpackages="gmpxx gmpxx-devel gmp-devel"
+
 if [ "$CROSS_BUILD" ]; then
 	hostmakedepends+=" libtool"
 	pre_configure() {
@@ -28,8 +30,17 @@ gmpxx_package() {
 		vmove usr/lib/libgmpxx.so.*
 	}
 }
+gmpxx-devel_package() {
+	depends="gmpxx-${version}_${revision}"
+	short_desc+=" - C++ development files"
+	pkg_install() {
+		vmove usr/include/gmpxx.h
+		vmove usr/lib/libgmpxx.a
+		vmove usr/lib/libgmpxx.so
+	}
+}
 gmp-devel_package() {
-	depends="gmpxx>=${version}_${revision} gmp>=${version}_${revision}"
+	depends="gmp-${version}_${revision}"
 	short_desc+=" - development files"
 	pkg_install() {
 		vmove usr/include

srcpkgs/gmpxx-devel

@@ -0,0 +1 @@
+gmp